Mechanism of Substitution Reactions

Some chemical reactions involve the substitution of an atom or a group of atoms by some other atoms or group of atoms without any change in the structure of the remaining part of the molecule. These reactions are referred to as substitution reactions.

Depending upon the nature of the attacking species, the substitution reactions can be of three types, nucleophilic, electrophilic and free radical substitution reactions.

Nucleophilic substitution reactions are the reactions which involve the attack of a nucleophile.

Some basic stereochemical principles and notations such as optical activity, plane polarised light, asymmetric carbon, chirality, racemic mixture, retention, inversion, racemisation, enantiomerism, superimposability, optical inactivity etc. help to understand nucleophilic substitution reactions in a better way.

Based on rotation of the plane of plane polarised light, enantiomers may be dextrorotatory (d or +) or laevorotatory compound (l or –). An optical isomer can be named according to spatial configuration of its atoms as D and L.

Nucleophilic substitution reactions, on the basis of their mechanism and kinetic properties can be of two types, SN1 reactions and SN2 reactions, where S stands for substitution; N stands for nucleophilic, 1 stands for unimolecular and 2 stands for bimolecular.

SN1 reaction is a two-step reaction, which proceeds via the carbocation intermediate and via racemisation.

Polar protic solvent having the high dielectric constant increases the rate of SN1 reaction.

SN2 reaction is a single-step reaction and proceeds via the transition state.

SN2 reactions proceed via inversion of the configuration (Walden inversion).

Aprotic solvent and icreasing nucleophilic strength favour SN2 Mechanism.

Mechanism of electrophilic substitution reactions involves generation of electrophile, formation of carbocation intermediate and formation of product.

Some common electrophilic substitution reactions are Nitration, Halogenation, Sulphonation, Friedel–Crafts alkylation or acylation.

Free radical substitution reactions are brought about by the attack of free radicals.

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